1. Field of the Invention
This invention relates to the reduction of crosstalk in an optical network and in particular concerns the reduction of interferometric noise such as incoherent beat noise.
2. Related Art
Interferometric noise may occur when an optical wave sheds a fraction of its power, which later interferes with its parent after having experienced a temporal delay. When the temporal deiay greatly exceeds the source coherence time, the interferometric noise is termed incoherent beat noise. If this condition is not met, the noise is partially coherent or coherent in nature. Several architectures in which such interference takes place have been identified, including delay elements in all-fibre optical bit rate limiters, reflections and Rayleigh backscatter in multi-stage amplified links, and crosstalk in wavelength division multiplexed (WDM) and optical time division multiplexed (OTDM) switching networks.
OTDM switching networks have been demonstrated to support high bandwidth traffic, and are additionally transparent to bit rate, coding format and wavelength. They may be configured to switch continuous high bit rate services such as high definition television (HDTV) or bursty traffic in asynchronous transfer mode (ATM) cells. OTDM switching networks employing amplitude shift keying (ASK) digital transmission/direct detection may be constructed from 2.times.2 switching elements interlinked by optical fibre delay lines (see the specification of PCT/GB92/00400). Inter-channel crosstalk arises in such networks because, in practice, the crosspoints do not possess perfectly isolated outputs. Every TDM channel emerging from the network is corrupted by the unwanted crosstalk waveforms.
In dealing with such unwanted crosstalk, known systems have concentrated on minimising incoherent noise-free crosstalk. The degradation resulting from this crosstalk may be modelled by a "sum of intensities" approach.